Technical Field
The present disclosure relates generally to measuring a species in a gas, and more specifically to methods and apparatus for measuring concentration of a gaseious or particulate species using photoacoustic measurement of light adoption.
Background Information
Particulate matter (PM) refers to microscopic solid or liquid matter suspended in a gas, for example, air. One category of PM is atmospheric PM, (e.g., black carbon soot, atmospheric aerosol, and other species) that are produced as a byproduct of combustion, industrial processes or certain natural processes. Atmospheric PM can significantly impact human health. For example, atmospheric PM may be harmful if inhaled, leading to increase rates of lung cancer among other health problems. Likewise, atmospheric PM may absorb significant amounts of solar energy, promoting global climate change. For instance, black carbon soot is highly effective at absorbing solar energy, absorbing about a million times more energy per unit of mass than carbon dioxide (CO2).
Another type of PM is commercial nanoparticles (e.g., ceramic silicon carbide nanoparticles, polymeric micelle nanoparticles, platinum-cobalt nanoparticles, and other species) that are synthesized for applications in manufacturing, biomedicine, optics, electronics and other fields. The properties of many conventional materials change when formed from very small particles, in part due to an increase in the ratio of surface area to weight that renders them more reactive. Such increase in reactivity has caused a growing interest in the production of commercially desirable PM species.
For both atmospheric and commercially produced PM there is often a need to measure the concentration of a PM species in a gas. One common measurement technique utilizes the photoacoustic (PA) effect. The PA effect is a process of acoustic wave generation from absorption of light. The basic theory behind the PA effect is that radiation energy from the light absorbed by a species of PM in a sample gas is released through non-radiative relaxation by generating heat in a localized region and creating an acoustic wave. Light absorption is normally proportional to the concentration of the species according to the Beer-Lambert law, which can be expressed as:A=σ(λ)Lc, where A is a measure of absorbance, σ(λ) is a wavelength-dependent absorptivity coefficient, L is the path length, and c is the concentration of the absorbing species. If τ(λ) and L are known, then by measuring absorbance, concentration of the species of PM may be determined.
A number of PA effect measurement instruments have been developed to attempt to measure concentration of species of PM in a sample gas. However, existing instruments generally suffer a number of shortcomings. First, existing instruments often experience high levels of interference from background absorption, background vibrations (i.e. background noise) and wall effects. Typically, such instruments employ a single sample cell in which the sample gas is subject to light, such that both the species of PM and gaseous species (e.g., nitrogen dioxide (NO2)) in the sample contribute to the resulting acoustic wave. Further, background vibrations incident upon the sample cell from the surrounding environment are incorporated into the acoustic wave. Likewise, the effects of radiation energy on the walls of the sample cell further influence the acoustic wave.
Second, exiting instruments typically have lower than desired sensitivity, require significant volumes of sample gas, and have longer-than desired response times. A number of factors contribute to these issues, including the use a single microphone with limited sensitivity to detect the acoustic wave, and the use of a light source that has significant beam divergence requiring larger sized sample cells.
Third, existing instruments typically are sensitive to temperature, composition and humidity of the sample gas. Further, they often are sensitive to pressure change effects associated with flow of the sample gas. Such instruments often place their microphone in direct contact with the sample gas, resulting in interference and limiting response rate.
Accordingly, there is a need for improved apparatus and methods for measuring concentration of a species (e.g., a species of PM or, alternatively, a gas-phase species) using measurement of light absorption that may address some or all of these shortcomings.